Previous work from our laboratory has led to the conclusion that in the Ehrlich-Lettre ascites tumor cell both chloride and sulfate are transported by a common mechanism possessing two reactive membrane sites. This conclusions is based on: (1) the effect of sulfate on chloride transport and chloride on sulfate transport; and (2) the effect H2DIDS (4,4'-diisothiocyano-1,2 diphenyl ethane, 2,2'+disulfonate) on the transport of both anions. Reversibly bound H2DIDS inhibits transport of sulfate to a greater extent than chloride. However, covalently bound H2DIDS inhibits transmembrane movement of sulfate but is without effect on chloride. This suggests that H2DIDS interacts irreversibly with that component of the anion transporter concerned with the regulation of sulfate permeability. We have recently synthesized tritium labeled-H2DIDS. Consequently, in the coming year the relationship between (3H)H2DIDS binding and inhibition of sulfate transported will be determined. Cells will be fractionated and the plasma membranes will be isolated. The association of labeled (3H)H2DIDS with membrane protein will be determined using polyacrylamide gel electrophoresis.